1. Field of the Invention
This invention pertains generally to the field of navigation systems such as, but not limited to, aircraft navigation systems.
2. Description of the Related Art
The continuing growth of aviation has placed increasing demands on airspace capacity and emphasizes the need for the best use of the available airspace. These factors, along with the accuracy of modern aviation navigation systems and the requirement for increased operational efficiency in terms of direct routings and track-keeping accuracy, have resulted in the concept of “Required Navigation Performance” (“RNP”) standards—statements of the navigation performance accuracy necessary for operation within a defined airspace. Some of these standards appear in an Advisory Circular (“AC”) 90-101A published by the Federal Aviation Administration (“FAA”) and in a Document (“DO”) published by the Radio Technical Commission for Aeronautics (“RTCA”). For example, the FAA has published AC 120-29A entitled “Criteria for Approval of Category I and Category II Weather Minima for Approach,” and the RCTA has published DO-236B entitled “Minimum Aviation System Performance Standards: Required Navigation Performance for Area Navigation.”
RNP may include performance and functional requirements indicated by the RNP type. The RNP type defines the total system error (“TSE”) that may be allowed in lateral and longitudinal dimensions within a particular airspace, where the TSE takes into account path definition errors (“PDE”), navigation system errors (“NSE”), and flight technical errors (“FTE”). The RNP type is used to specify navigation requirements for the airspace or a navigation system that provides a specified level of accuracy defined by a lateral area of confined airspace in which an RNP-certified aircraft operates. For example, an RNP 0.3 level requires a normal navigational accuracy of 0.3 nautical miles (“NM”) at least 95 percent of the total flight time; similarly, an RNP 0.1 level requires a normal navigational accuracy of 0.1 NM at least 95 percent of the total flight time. Hence, a lower level of RNP means a greater level of navigational accuracy. With the availability of satellite navigation systems such as the Global Positioning System (“external source”), future airspace systems may rely heavily on external source-based navigation to which stringent standards for NSE may be applied. An instrument approach procedure (“IAP”) specifying an RNP level of less than RNP 0.3 or lower than the standard minima at airports not equipped with Instrument Landing Systems (“ILS”) airports may require a method to detect external source position errors before the 6.2 second time-to-alert standard that currently exists. A similar requirement may be made for manufacturers and/or end-users wishing to take advantage of the standards published in RTCA DO-315B entitled “Minimum Aviation System Performance Standards (MASPS) for Enhanced Vision Systems, Synthetic Vision Systems, Combined Vision Systems and Enhanced Flight Vision Systems.”
AC 90-101A prescribes one method to comply with RNP<0.3 requirements. This method's system architecture requires dual FMS, dual GPS, dual FD or autopilot and a single IRS as the minimum architecture for a RNP<0.3 system. This minimum architecture provides the redundancy required to meet the probability of presenting misleading information to the pilot or flight crew of 1*10^(−7). To meet the desired probability of misleading information requires both sides of a dual system be compared by the system itself or by the pilot to detect when a failure occurs. AC90-101A, also, requires the loss of all lateral deviation be equal to or less than 1*10^(−7). The achievement of this requirement is not readily available in a dual-channel system because the channel-to-channel comparison required to meet the architectural requirement may also result in a failing of both channels because it may not readily apparent to the system, pilot, or flight crew which data source has failed and which one is acceptable. To meet the requirement, the integrity of each channel in the dual-channel system may be determined independently of the other. Furthermore, the detection of the favorable channel could be considered important to the pilot who is executing one or more Radius-to-Fix (RF) legs because an aircraft flying at 180 knots will fail to meet the requirements of RNP 0.1 in 4 to 6 seconds after a failure. This may be an unacceptable amount of time in which to determine an unfavorable channel.